1. **Difference between single welded and double welded pipe:**
Single welded pipe has one longitudinal weld seam joining the edges. Double welded pipe has two parallel longitudinal seams 180 degrees apart. Double welding improves circularity and pressure integrity but increases manufacturing complexity and cost.
2. **Welded pipe vs. SMLS (Seamless) pipe:**
- **Welded pipe** is cheaper, available in larger diameters, but has a weld seam that *can* be a potential weak point.
- **SMLS pipe** has no weld seam, offering uniform strength, better pressure containment, and corrosion resistance. It's stronger but costlier and size-limited.
**Better?** Depends on use: SMLS is superior for high-pressure/temperature (e.g., oil & gas risers); welded suffices for structural/low-pressure applications (e.g., water pipelines).
3. **Two different ways of welding pipe:**
- **Longitudinal Welding (LSAW/ERW):** Steel plate edges joined lengthwise to form a straight seam. Common for pipes 10+ inches in diameter.
- **Spiral Welding (HSAW):** Steel coil helically wound and welded diagonally. Allows continuous production of large diameters with balanced stress distribution.
4. **Difference between single weld and double weld:**
- **Single weld:** One welding pass completes the joint. Faster but may lack penetration or strength in thick materials.
- **Double weld:** Two sequential passes (e.g., root + cap). Ensures full penetration, reduces defects, and improves mechanical properties for critical applications.
5. **Minimum gap between two welds:**
The minimum distance is typically **6 times the pipe wall thickness (6t)** or **50 mm (2 inches)**, whichever is larger. This prevents overlapping heat-affected zones (HAZs) that could weaken the material. Exceptions exist for specific codes (e.g., ASME B31.3 allows reduced gaps with engineering review).
